A sound generating apparatus is used to allow a pedestrian to know the status inside an intersection by means of speech information, for example.
For example, a warning sound generating apparatus disclosed in Patent Literature 1 emits two types of ultrasound waves having different frequencies from different fixed parts, respectively, and allows a pedestrian present in an area in which the two types of ultrasound waves overlap with each other to hear a sound alarm. Specifically, the warning sound generating apparatus emits the first ultrasound wave (frequency fa) and the second ultrasound wave (frequency fb different from frequency fa) from two generators placed on street sides, respectively, and generates an audible sound having frequencies fa-fb in the area in which the first ultrasound wave and the second ultrasound wave overlap with each other. A principle of generating the audible sound is based especially on a “principle of a parametric speaker with reference to a frequency shift between two types of ultrasound waves.” Accordingly, allowing only the pedestrian present in a fixed area in which the first ultrasound wave and the second ultrasound wave overlap with each other to hear necessary information is possible.
In addition, an attention calling apparatus disclosed in Patent Literature 2 emits an ultrasound wave having a single frequency, and allows a pedestrian present in a specific area to hear a sound alarm. Specifically, the attention calling apparatus includes an ultrasonic sonar, and is mounted on a moving object (e.g., vehicle). The attention calling apparatus emits an ultrasound wave (i.e., frequency fd), which is generated by performing amplitude modulation on ultrasound waves (i.e., frequency fc) with audible signals (i.e., audio signals), at a sound pressure level of 110 dB or more to a predetermined region, and generates the audible sound within an area covered by the generated ultrasound wave. The principle of generating this audible sound is based especially on a “principle of a parametric speaker with reference to nonlinear characteristics when an ultrasound wave propagates through the air.” Accordingly, for example, it is possible to allow only a pedestrian who enters a dangerous area in front of a vehicle to hear warning information.